This invention relates to thermally responsive trigger devices suitable for use in liquid metal (conveniently identified as sodium) cooled systems such as sodium-cooled fast reactors where a trigger may typically be required at temperatures up to 700.degree. C. to release neutron absorber units.
"Hot" magnetic triggers are known. Such devices rely upon the fact that all magnetic materials have a Curie point which is a temperature at which the material becomes de-gaussed and ceases to function magnetically. The attainment of the Curie point can, for example, allow the release of a component which had previously been magnetically held or retained. For such triggers to be acceptable a suitable magnetic material has to be selected to give the trigger function at the correct temperature and at the same time the selected material must be acceptable in the context of its use, and not deteriorate or introduce risks. Such devices are not adjustable and they also operate under stress.
Fusible links are also known but these can give rise to problems similar to those referred to above in relation to magnetic triggers.
Bellows compression devices are also known (see for example GB-PS 1,580,322) in which an expansible material such as sodium-potassium alloy contained in a helical tube, is connected with a reservoir column outside a bellows to compress the bellows with temperature rise and the bellows then operates a trigger via linkages. Such devices include a significant number of co-related parts and hence there must be a modest probability of failure of the parts themselves or at the inter connection between the parts. Repair or replacement of failed parts in a nuclear reactor can be very difficult and costly.